1. Field of the Invention
The present invention relates to a fuel injector for an internal combustion engine.
2. Description of Related Art
Fuel injectors eject pressurized fuel into the combustion chamber of an internal combustion engine. FIG. 1 shows a fuel injection system 1 of the prior art. The injection system includes a nozzle 2 that is coupled to a fuel port 3 through an intensifier chamber 4. Flow through the nozzle 2 is controlled by a needle valve 5 that is biased into a closed position by a spring 6. The injector 1 contains an intensifier piston 7 that increases the pressure of the fuel in the chamber 4. The pressurized fuel lifts the needle valve 5 to allow injection of fuel into the combustion chamber of an engine.
The intensifier piston 7 is hydraulically driven by a working fluid that flows through port 8 and controlled by a poppet valve 9. The poppet valve 9 is coupled to a actuator 10 that can be energized to pull the valve 9 into an open position. As shown in FIG. 2, when the actuator 10 opens the poppet valve 9, the working fluid flows through the port 8 and applies a pressure to the intensifier piston 7. The pressure of the working fluid moves the piston 7 and pressurizes the fuel. When the actuator 10 is de-energized, springs 11 and 12 return the poppet valve 9 and the intensifier piston 7 back to the original positions, respectively.
Spring return fuel injectors are relatively slow because of the response time of the return springs. Additionally, the springs generate an additional force which must be overcome by the actuator. Consequently the actuator must be provided with enough current to overcome the spring force and the inertia of the valve. Higher currents generate additional heat and degrade the life and performance of the actuator. Furthermore, the spring rate of the springs may change because of creep and fatigue. The change in spring rate will create varying results over the life of the injector. It would therefore be desirable to provide a fuel injector that contains no springs.
U.S. Pat. No. 5,460,329 issued to Sturman discloses an hydraulically driven fuel injector. The Sturman injector includes a actuator actuated fluid control valve which controls a flow of hydraulic fluid that drives an intensifier of the injector. In one position the fluid control valve allows the hydraulic fluid to flow into the intensifier. The flow of hydraulic fluid drives the intensifier and ejects fuel from the injector. When the control valve is actuated into a second position the intensifier moves back to the original position and drains fuel into the injector.
The fluid control valve of the Sturman injector is either a single three-way valve or a single four-way valve. The control valve is actuated between two positions. In one position the control valve allows the hydraulic fluid to flow from a supply port into a cylinder passage to drive the intensifier. In the other position the control valve couples the cylinder passage to a drain line so that the intensifier moves back to the original position. The control valve therefore has only two modes of operation, driving the intensifier and returning the intensifier. It would be desirable to provide a valve assembly for a hydraulically driven injector that has more than two modes.
For example, the hydraulic fluid for hydraulically driven fuel injectors is typically provided by a pump and a series of fluid lines. The fluid system typically contains a spring biased pressure relief valve which opens to insure that the fluid pressure does not exceed a certain level. The pump must perform work to overcome the spring of the relief valve during the by-pass mode of the system. It would be desirable if the valve assembly of the injector could also provide a by-pass function for the hydraulic system.
Additionally, the fluid control valves are actuated to either drive the intensifier to eject fuel, or return the intensifier to draw more fuel into the fuel chamber of the injector. It may be desirable to maintain the position of the intensifier at an intermediate position to more accurately control the amount and rate of fuel injection. None of the valve assemblies of the prior art allow an intensifier to be held in an intermediate position.